Removing Optical Distortions

visitors since October 13, 1998

Pictures taken through a teleconverter or a monocular can suffer from "pincushion" distortion -- vertical and horizontal lines appear to bow inwards towards the center of the image. This page shows how to remove the distortions introduced by add-on lenses using the spherical distortion feature in Photoshop 5. The example on this page work well for pictures taken with an Epson PhotoPC 600 and a monocular. The specifics may vary for different cameras and teleconverters/monoculars, but the general procedure should be the same.

1. Open the image in Photoshop.
   
   
2. Increase the canvas size so that it is square and is 15-50% larger than the image's longer dimension. For my images I've found that 25% larger is optimal. Thus, I expand the canvas around a 1024x768 image to 1280x1280. Note: 1280 is 25% larger than 1024.
   
   
3. Use the Spherical distortion filter. (Filter|Distort|Spherize). Play with the degree of distortion until you get one that looks right. I've found that a setting of 28 works well for my images.
   
   
4. Resize the canvas to a size that eliminates the vignetted portion of the image. I've found that I can usually resize the image to 800x600, 840x630 or 880x660 depending on the original.
   
   

Here is a image that was taken using the monocular, before and after the Photoshop transformation:

Distorted image (cropped)

Corrected image (cropped)

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